Light bulbs have been constructed for many years with an evacuated glass enclosure or with a gas-filled glass enclosure having a filament positioned inside and sealed within the glass enclosure or bulb. Electrical terminals have been connected to the filament and sealingly secured through a base, which may include a compound of rubber, resin or plaster of paris molded around the electrical terminals. The base provided mechanical support for the filament within the bulb and for the terminals or connecting pins extending from the base. One early example of a light bulb with a support base is shown in U.S. Pat. No. 467,982. Also, from an early time, light bulbs have been attached to aircraft wings, as disclosed in U.S. Pat. No. 1,619,100, which discloses a rudimentary method for securing a bulb to an aircraft wing. In particular, a rubber strap is shown to have been used for the purpose of securing a light bulb to an airplane wing.
Incandescent light bulbs now must operate reliably for long periods of time in a particularly high vibration environment of high-speed, jet-propelled aircraft. Aircraft manufacturers have partially addressed some of the difficulties associated with high vibration by using fluorescent bulbs whenever possible, as for example, in passenger seating and cockpit areas. However, fluorescent bulbs are not acceptable for all aircraft purpose because of their physical size limitations which require sufficient mounting area, and also because the light output from fluorescent bulbs is not always adequate.
High output lighting applications are addressed with the use of incandescent bulbs, and typically quartz-halogen bulbs, either with an integral reflector, as for example, with landing lights, or bare, non-reflectorized bulbs which are mounted in separate mounting assemblies, as for example, marker, navigation and position lights. The non-reflectorized bulbs are typically connected to aircraft power through insulated wire leads, which are attached, as by spot welding, to short terminal pins which connect to the bulb filament. Aircraft users have reported high failure rates with non-reflectorized bulbs due to wire lead breakage and/or shorting at the base of the bulb. In the past, the high operating temperature of the quartz-halogen bulbs, reportedly greater than about 200.degree. C. and as high as about 300.degree. C. or higher when in an enclosed area, as well as potential for exposure to chemicals such as hydraulic fluids, lubricating oils, jet engine fuel and anti-icing, de-icing and defrosting fluids has hindered prior attempts to insulate the wires from shorting. Further, insulation alone does little to reduce the high failure rate due to inadvertent excess strain on the pins and wire connectors or fatigue failure to excessive vibration.
In other situations where bulb assemblies have needed to operate in underwater or submerged conditions and associated environments where the lamp may be subjected to extreme temperatures and various corrosive conditions, total encapsulation of the bulb has been disclosed, as in U.S. Pat. No. 3,946,263. According to this disclosure, a silicon rubber coating is sprayed around the entire bulb, including on the electrical terminals for providing waterproofing. After the silicon rubber is applied, then a boot is filled with additional silicon rubber and is secured over both of the bulb terminals simultaneously. The silicon rubber composition is allowed to cure such that the bulb is substantially watertight. However, while the silicon rubber provides electrical insulative characteristics and also provides some resistance to certain types of corrosive environments, such as water, the silicon rubber continues to remain flexible and does not provide the type of rigid mechanical separation between the wire terminals which will adequately prevent inadvertent strain. Further, the flexible silicon rubber continues to allow the wire to vibrate within the rubberized silicon at the high frequencies sometimes present with jet turbine aircraft engines.
There continues to be a need for vibration-resistant, high temperature halogen light bulbs, and also, for a method of constructing such bulbs. There is a particular need for a method of retrofitting existing light bulbs which are currently used in specific types of mounting devices found in use as wingtip position indicator lights in a large number of existing aircraft. Particularly, it has been found that the failure rate, for light bulbs currently used as wing position indicator lights in large fleets of MD80's, is very high, resulting in a maintenance schedule which must be accelerated due to a terminal or connection failure rate, which failure rate is higher than the normal maintenance schedule which is designed to address the filament expected failure rate.